Module io/stdio

Typed standard-stream handles — stdin() / stdout() / stderr() (plans/archive/STD_API_AUDIT.md §D5).

Each handle wraps its well-known file descriptor (0/1/2) and implements the async Reader/Writer traits from std/io, so code written against the traits works on the standard streams the same way it works on a File or a TcpStream. This is the typed replacement for the BufReader.new(i32(0)) magic number.

The handles are UNBUFFERED: every read/write is one async syscall, and they keep NO offset of their own. Each transfer passes offset u64.MAX, which the runtime reads as "the descriptor's own position" (read(2)/write(2), not pread/pwrite; std/sys/file's offset rules). So every stdout() handle, every stderr() handle redirected onto the same file (2>&1), libc's stdout and a parent shell's earlier writes all share the kernel's one file position: a stream redirected to a regular file is appended to, never overwritten (issues/fixed/stddoc-io-stdio-handles-write-positionally-and-clobber-redirected-output.md).

Sharing the position is not sharing a buffer: std/fmt's print and println write through libc's buffered stdout, which flushes when its buffer fills or at exit, so text printed before a handle write can land AFTER it (no bytes are lost). Flush libc first (std/libc/stdio's fflush) when the order across the two writers matters.

There is no write_string here: the handles carry exactly the trait surface, whose write/write_all take a raw *(u8), so writing text from safe code goes through BufWriter(Stdout) (which has write_string/write_bytes) or through std/fmt. Likewise the line primitive for input is BufReader(Stdin).read_line, not a method here.

Stability

unstable — Rust's io::stdout() returns a handle to ONE shared, locked stream that println! also writes through, while stdout() here returns an independent, unbuffered object and std/fmt keeps libc's own buffer (the ordering note above). Joining the two writers is the open question. The three types, the three accessors and the fd() methods are expected to keep their names.